1. Field of the Invention
The present invention relates to a multichannel amplifier having an amplifying unit for a headphone.
2. Description of the Related Art
A conventional multichannel amplifier will be described. FIG. 3 is a circuit diagram showing a schematic configuration of the conventional multichannel amplifier. The conventional multichannel amplifier has a volume control unit 3a for a front channel, a volume control unit 3b for a surround channel, an amplifying unit 4a for a front channel, an amplifying unit 4b for a surround channel, an amplifying unit 4c for a headphone, a control unit 5, an output terminal 6 for a front speaker, an output terminal 7 for a surround speaker, an output terminal 8 for a headphone, and switches SW1 to SW3.
To the front channel volume control unit 3a, a front channel signal FL/FR as an analog audio signal is inputted via an input terminal 1. The front channel volume control unit 3a controls the level of the front channel signal FL/FR and outputs the resultant signal to the front channel amplifying unit 4a. The front channel amplifying unit 4a amplifies the front channel signal FL/FR and outputs the amplified signal to the front speaker output terminal 6 via the switch SW1. The front channel signal FL/FR is outputted to a front speaker connected to the front speaker output terminal 6 and converted to sound.
To the surround channel volume control unit 3b, an analog surround channel signal SL/SR is inputted via an input terminal 2. The surround channel volume control unit 3b controls the level of the surround channel signal SL/SR and outputs the resultant signal to the surround channel amplifying unit 4b. The surround channel amplifying unit 4b amplifies the surround channel signal SL/SR and outputs the amplified signal to the surround speaker output terminal 7 via the switch SW2. The surround channel signal SL/SR is outputted to a surround speaker connected to the surround speaker output terminal 7 and converted to sound.
An input terminal of the headphone amplifying unit 4c is connected in parallel with the front channel amplifying unit 4a at the rear stage of the front channel volume control unit 3a. The front channel signal FL/FR whose level is controlled by the front channel volume control unit 3a is inputted to the headphone amplifying unit 4c. The headphone amplifying unit 4c amplifies the inputted front channel signal FL/FR and outputs the amplified signal to the headphone output terminal 8 via the switch SW3. The front channel signal FL/FR is outputted to a headphone connected to the headphone output terminal 8 and converted to sound.
When the headphone is connected, the headphone output terminal 8 outputs a connection signal indicative of the connection between the headphone and the headphone output terminal 8 to the control unit 5. The control unit 5 detects the connection signal outputted from the headphone output terminal 8 and, based on the connection signal, turns on/off the switches SW1 to SW3.
In a case where no headphone is connected to the headphone output terminal 8, the control unit 5 turns on the switches SW1 and SW2 and turns off the switch SW3 so that the front channel signal FL/FR and the surround channel signal SL/SR are outputted to the front speaker and the surround speaker, respectively. In a case where the headphone is connected to the headphone output terminal 8, the control unit 5 turns off the switches SW1 and SW2 and turns on the switch SW3 so that the front channel signal FL/FR is outputted to the headphone.
In such a conventional multichannel amplifier, even in a case where no headphone is connected to the headphone output terminal 8, the headphone amplifying unit 4c is connected in parallel with the front channel amplifying unit 4a at the rear stage of the front channel volume control unit 3a. There is, consequently, a problem such that the input impedance of the front channel amplifying unit 4a drops and a wire to the headphone amplifying unit 4c becomes an inductor component, thereby deteriorating the sound quality.
Consequently, a relay is provided at the front stage of each of the front channel amplifying unit 4a and the headphone amplifying unit 4c. When no headphone is connected to the headphone output terminal 8, by turning on/off the relays, the front channel amplifying unit 4a and the headphone amplifying unit 4c are set to an electrically-not-connected state, and the problem is solved. However, the relays are expensive, so that a problem of increase in cost occurs.